Planetary/Atmospheric volumetric clouds
A 3D cloud effect with shadows on a planet can be generated using the experimental micro-displacement with adaptive subsurfaces and shader displacement. Using subsurface scatter and transparency in the shader given an appearance similar to a volumetric shader.





Procedure (tested on blender 2.79b)


Create the planet by adding a sphere to the scene. This will be the planet surface. 
Add a second sphere for the clouds. This sphere should be slightly larger than the planet surface. The distance between the cloud sphere and the planet sphere will be the height of the lowest clouds.


Add a subsurface modifier to the cloud sphere. Enable the 'Adaptive' option. 

Enable experimental feature support in the render panel.

Create an image for the cloud height

Create an image for cloud coverage. This image will be used to set the transparency used for the cloud shader. If you look carefully, you'll see that the cloud coverage in this example is set to cover more area than the image for cloud height.

Enable cycles and create a shader for the clouds. The cloud material is a mix of a subsurface scatter and a diffuse nodes. This is mixed with a transparency node using the cloud coverage image. The cloud height image is connected to the displacement input on the material output node.





Without displacement, the clouds are flat.

With displacement, adaptive subdivision, and experimental features, the clouds become 3D. 





If clouds with overhangs are needed, multiple layers of clouds can be used. 



Blend file: